For example, FIG. 6 of the accompanying drawings shows a manual hydraulic pump 50 as a prior art of the foregoing type. The pump 50 has a pump housing 51 consisting of a front housing 52 and a rear housing 53, which is fixed to the rear end of the front housing.
A shaft 54 extends through and is supported rotatably by the front housing 52. An inclined thrust bearing 60 is fitted in the front housing 52 and surrounds the shaft 54. The pump housing 51 houses a cylinder block 55, which is supported rotatably by the rear housing 53. The cylinder block 55 is fixed to the shaft 54 to rotate with it and has cylinders 56 open at their front ends. A cylindrical piston 57 (hatched in FIG. 3) can slide in each cylinder 56. The cylindrical piston 57 has a spherical head formed at its front end and is open at its rear end. A spring seat 58a is put in the cylindrical piston 57. A coil spring 58 is interposed between the spring seat 58a and the bottom of the associated cylinder 56 to bias the cylindrical piston 57 forward into compressive contact with the thrust bearing 60.
When the cylinder block 55 is turned with the shaft 54 in one direction, the cylindrical pistons 57 slide forward and backward to create hydraulic pressure. Specifically, one or more cylindrical pistons 57 slide backward against the force of the associated springs 58 to discharge hydraulic oil through the discharge port of the pump 50. At the same time, the other cylindrical pistons 57 are slid forward by the associated springs 58 to suck hydraulic oil through the suction port of the pump 50.
The discharge and suction of hydraulic oil are controlled by a distributing valve 59, which is fixed to the rear housing 53 by screws 59a. The cylinder block 55 turns with its rear end in contact with the distributing valve 59. A similar structure is disclosed in Japanese unexamined patent publication No. H11-222197 (paragraph 0017 and FIG. 3).
The operation of the pump 50 requires that each cylindrical piston 57 be biased forward by the associated spring 58, which is fitted in the associated cylinder 56. As a result, the pump 50 has a large number of parts and is complex in structure.
Air accumulates in the cylindrical pistons 57 and is not liable to escape from them. When the cylindrical pistons 57 slide backward, the air in them is compressed. As a result, hydraulic oil may be discharged intermittently from the pump 50, so that the pumping action may be intermittent.
As is the case with conventional pumps of this type, the distributing valve 59 is a disc. As stated already, the distributing valve 59 is fixed to the rear housing 53, and the cylinder block 55 turns in contact with this valve. Considerably high hydraulic pressure is exerted on the distributing valve 59. For this reason, in order to prevent leakage of hydraulic oil, it is necessary to finish with great accuracy the rear end surface of the cylinder block 55, the front and rear surfaces of the distributing valve 59, and the surface of the rear housing 53 that is in contact with the valve.